1. Cross-Reference to Related Application
U.S. patent application Ser. No. 176,202 entitled "Microwave Measurement System And Associated Method", invented by Martin I. Grace et al, filed on 3/31/88, and owned currently and at the time of invention by a common assignee, is incorporated by reference.
2. Field of the Invention
The present invention relates to power supplies in general and in particular to a switching power supply with an injection signal frequency locking circuit for use in a network analyzer.
3. Description of the Prior Art
A network analyzer is an apparatus for measuring the performance of a network, i.e. a device under test (DUT), over a predetermined range of frequencies. The typical network analyzer comprises a high gain receiver which is sensitive to signals as low as -100 db.sub.M(50) (the voltage across a 50 ohm resistor dissipating 1 milliwatt of power).
A high gain receiver of the type used in a network analyzer typically comprises several intermediate frequency stages and frequency passbands. Because of the extremely high sensitivity of such receivers, extraneous radio frequency signals which fall within the passbands of the receiver will interfere with the measurements being made.
Power supplies are required in a network analyzer to provide power to operate the various electronic circuits located therein. To eliminate the power supply as a source of radio frequency interference, it has been the practice to use linear power supplies which operate at a relatively low frequency, e.g. 60 Hz.
The typical network analyzer is a relatively high powered device requiring as much as 200 watts of power which must be provided at various potentials. As a consequence, the linear power supply used in a typical network analyzer comprises a relatively large transformer which is quite heavy and requires a great deal of space. Moreover, because the typical linear power supply is relatively inefficient, e.g. 50%, it dissipates a great deal of heat which must be removed from the apparatus.
A switching power supply is another type of power supply which in comparison with a linear power supply having the same power rating, is much more efficient, e.g. 90%, is lighter in weight, requires less space and generates relatively little heat.
A typical switching power supply comprises a transformer having a primary winding across which a potential is applied via a switching circuit at a relatively high frequency. The high switching frequency has the advantage of significantly reducing the size and weight of the transformer. However, it also has the disadvantage of generating radio frequency signals which can interfere with signal processing in radio frequency sensitive receivers. As ambient conditions change, the switching frequency may also change. In most applications, a change in the switching frequency is usually not a problem because the potential output of the supply is not primarily dependent on switching frequency and is regulated by negative feedback. However, in applications comprising radio frequency sensitive receivers, the radio frequency signals generated in a switching power supply can be troublesome, particularly if their frequency changes with changes in ambient conditions and they fall within the passbands of the receiver. For this reason, heretofore, linear power supplies have been the supply of choice for network analyzers, not withstanding their above-described disadvantages.